Fix admin CLI system variable commands (#14956)

## What

Fixes #12409.

Implements admin CLI support for:

- `list vars;`
- `show var <name-or-prefix>;`
- `set var <name> <value>;`

## Changes

- Wire Go CLI variable commands to the admin API.
- Support integer and quoted string values in `SET VAR`.
- Return variable rows as `data_type`, `name`, `setting_type`, and
`value`.
- Add exact-name lookup with prefix fallback for `SHOW VAR`.
- Validate values by stored data type: `string`, `integer`, `bool`, and
`json`.
- Keep the legacy Python admin CLI/server behavior aligned.
- Update admin CLI docs and add focused tests.

## Verification

- `go test -count=1 ./internal/cli`
- `python3.12 -m py_compile admin/server/services.py
admin/server/routes.py api/db/services/system_settings_service.py
admin/client/parser.py admin/client/ragflow_client.py`
- Python admin CLI parser smoke test for `SET VAR`, quoted values, `SHOW
VAR`, and `LIST VARS`.
- Attempted `./run_go_tests.sh`; local environment is missing native
tokenizer/linker artifacts:
  - `internal/cpp/cmake-build-release/librag_tokenizer_c_api.a`
  - `-lstdc++`

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

api/db/services/system_settings_service.py

@@ -26,7 +26,13 @@ class SystemSettingsService(CommonService):
     @classmethod
     @DB.connection_context()
     def get_by_name(cls, name):
-        objs = cls.model.select().where(cls.model.name == name)
+        objs = cls.model.select().where(cls.model.name == name).order_by(cls.model.name.asc())
+        return objs
+
+    @classmethod
+    @DB.connection_context()
+    def get_by_name_prefix(cls, name_prefix):
+        objs = cls.model.select().where(cls.model.name.startswith(name_prefix)).order_by(cls.model.name.asc())
         return objs
 
     @classmethod